Method of shaping an endo-prosthesis, a femoral head prosthesis, an acetabulum prosthesis and a method of fixing a femoral head prosthesis in a bone

ABSTRACT

According to the present invention, the stress patterns at the interface between bone and cement of an existing prosthesis are calculated by a finite element model. For this model the external load, material properties and boundary conditions are provided. By means of advanced digital computers, stresses and strains in the model are calculated. Desired stress patterns are achieved through an optimization procedure to determine that geometry which generates those stress patterns which approximate the desired ones best.

A first aspect of the present invention relates to designing prosthesesin general and more particularly femoral head protheses and acetabulumprostheses.

Prior art in this field comprises:

Article from Biomedizinische Technik, Vol. 30, No. 5, G. Giebel et al.:"Fertigung von Knochenmodellen nach Computer-Tomographie-Daten zurVerwendung in Chirurgie und Orthopadie" (pages 111-114);

Lecture during First European Congress of Knee Surgery and Arthroscopy,9-14 April 1984, Berlin, issued by E. L. Trickey et al. Springer-Verlag,Berlin, Heidelberg, DE U. Munzinger et al.: "Biomechanics of Kneeprostheses" (pages 324-334);

U.S. Pat. No. 4,153,953 (Grobbelaar), column 3, pages 19-28, 56-61;

DE-A-2805868 (Engelbrecht et al.), FIG. 1;

GB-A-2096003 (Burstein), page 2;

EP-A-0149527 (Lee et al.) FIGS. 1, 2

GB-A-2045082 (Raab), page 1

It is a first object of the method according to the present invention toimprove upon the prior art.

It is a further object to provide a method in which tensile, compressiveand shear stresses acting on the interface between prosthesis and bone,whether or not provided with a cement mantel, are controllable. Theoriesexist that resorption or necrosis of bone are caused by stress shieldingof the bone due to the prosthesis.

A preferred method according to the present invention concernsprostheses minimizing local normal and shear stresses between bone andprosthesis. Existing endoprostheses used in practice generally consistof a metal core surrounded by a mantle of an artificial material whichis in direct contact with the bone. According to the present invention,a mathematical simulation model is made of such a construction, by meansof the so-called finite element method. With this model the stresseswhich act on the connecting planes or at an interface area between theartificial mantle and the bone can be calculated. These stresses depend,amongst others, on the geometry of the prosthesis. By means of amathematical search procedure that geometry can be determined whichcreates a prechosen stress pattern at the interface between bone andprosthesis.

The above-mentioned method was applied to a 2-D model of a femoral headprosthesis and a acetabular prosthesis.

A second aspect of the present invention relates to a femoral headprosthesis having a specific shape.

The characteristics of this shape are not known from the prior art,e.g.:

DE-A-3.243.861; DE-A-2.805.868; BE-A-2.247.721; GB-A-2.078.523;GB-A+2.045.082; EP-A-146.192; EP-A-12.146; and U.S. Pat. No. 4,021,865.

A third aspect of the present invention is directed to a method for thepositioning of an endo-prosthesis in a bone as well as spacing orpositioning elements.

From the prior art a more cumbersome method for introducing a femoralhead prosthesis is known from U.S. Pat. No. 4,718,909.

According to a fourth aspect of the present invention an acetabularprosthesis having a characteristic shape is provided. Thosecharacteristic shapes are not known from the prior art, viz. U.S. Pat.Nos. 4,566,138 and 4,563,778.

Malfunctuning of a human joint due to arthrosis, arthritis, traumaconditions or any other cause reduces the well-being of the personconcerned and is expensive for society at large. With increasingsuccess, such defects are corrected by joint arthoplasty. Annually inthe world, these procedures are carried out more than 500,000 times.Most defects concern the hip and the knee joints; however, ankles, toes,shoulders, elbows, wrists and finger joints can also be provided withartificial joints.

The known prostheses usually consist of a combination of a metal and aplastic. Most prostheses have a metal stem or plateau which is fixedwithin the bone by means of acrylic cement. Acrylic cement is a fastcuring mixture of monomethyl-methacrylate liquid andpolymethylmethacrylate powder. This viscous mass can be introduced intothe intramedullary canal or into the trabecular bone of a bone.Subsequently, the prosthesis is inserted in the acrylic cement mass.After curing, a mantle of cement remains in between the metal core andthe bone. The bone cement does not have any adhesive properties, and theconnection between the cement and the bone is merely realised bymechanical interlock. A firm interlock is achieved by a sufficientlydeep penetration of the cement into the trabecular bone. Thispenetration arises if the acrylic cement has a sufficiently lowviscosity and is pressurized for some time.

In cases where no acrylic cement is used, the prosthesis is introduceddirectly into the bone. Such a prosthesis can be made out of metalcompletely, or can be composed out of a metal core and a coating. Inthis case there is no primary fixation between the prosthesis and thebone. Such a fixation can, however, be accomplished by the ingrowth ofbone into the surface structures of the prosthesis.

A well-known long-term complication of surgical joint replacement isloosening of the prosthesis. This loosening mostly occurs at theinterface between prosthesis and bone. It is known that the chances forthis complication to occur increase with decreasing age of the patientat the time of the operation and an accordingly increased activitylevel. Loosening of the prosthesis leads to resorption of bone andfinally to severe complaints of pain. These complaints can be sodramatic that a revision operation is indicated, in which the oldprosthesis is removed and a new one is inserted. However, the long-termresults of a revision operation are not as good as those of a primaryjoint replacement, and the revision procedure cannot be repeated anarbitrary number of times.

No common opinion exists presently on the cause or ethiology ofloosening. The following possibilities have been suggested: necrosis ofthe bone at the interface caused by toxicity of the cement monomer orexcessive heat release due to the exothermally curing bone cement,fracture of the cement mantle due to mechanical loading, failure of theinterface caused by local stress peaks, reduced loading of the bonecaused by stress shielding, etc.

Further advantages, features and details of the invention will beclarified by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic frontal view of a preferred design of a femoralhead prosthesis in accordance with the present invention.

FIG. 2 is a preferred design of the algorithm for the shaping of theprosthesis of FIG. 1.

FIG. 3 is a perspective, partly exploded view with cross-sections onseveral levels of a preferred design of a femoral head prosthesis inaccordance with the present invention.

FIG. 4 depicts five femoral head prostheses shaped in accordance withthe present invention.

FIGS. 5A-5G depicts seven acetabular prostheses shaped in accordancewith the present invention.

FIG. 6 a is a schematic view illustrating the procedure for the fixationof a femoral head prosthesis.

FIGS. 7, 8 and 9A-9F are views of a second embodiment of a femoral headprosthesis according to the present invention, showing a back view, sideview and section views resp.; and

FIG. 10 is an exploded view of a second embodiment of an acetabularprosthesis according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Loosening of a prosthesis can be caused by mechanical overloading of theinterface between acrylic cement and bone. Particularly meant here isthe occurrence of peaks in the normal and shear stresses at theinterface, and the occurrence of high strain energy density levels inthe cement at the interface. According to the present invention, thestress patterns at the interface between bone and cement of an existingprosthesis were calculated by means of a finite element model. In such amodel a particular geometry is approximated with a finite number ofelements. For this model the external load, the material properties andthe boundary conditions are provided as well. By means of advanceddigital computers stresses and strains in the model can be calculated.

The desired stress patterns are taken as the objective for amathematical search or optimization procedure to determine that geometrywhich generates those stress patterns which approximate the desired onesbest.

In a finite element model (FIG. 1) the elements 10, 11 and 12 representcortical bone, the element 3, 4 represent trabecular bone, the elements5, 9 (9') represent bone cement, and the elements 6, 7 and 8 representthe prosthesis. ##EQU1##

Next a nodal point -j- of the interface between the metal stem and thecement mantle is moved over a small distance, so a new geometry G_(j) iscreated. Of this new geometry the function f_(Gj) is calculated. This isdone for all nodal points -j-. The functions F_(Gj) obtained arecompared with F_(G). Depending on the decrease of the function F_(Gj)compared to the starting function F_(G), the geometry G is converted toG' by means of an algorithm 16--a so-called least-P alorithm--so thatF_(G) ' is smaller than F_(G). Subsequently, the geometry G' issubstituted via a feed-back loop 17 and the procedure is restarted. Whenthe desired geometry defined by criterium D is attained, the functionF_(G) ' will not decrease anymore and the procedure is terminated at 18.

Hereafter the shapes of the prostheses obtained with the above-mentioneddesign method will be shown, in particular a femoral head prosthesis andan acetabular prosthesis; it may be evident that the above-mentionedmethod is also applicable to other prostheses.

In the model shown in FIG. 1 the real three-dimensional structure wasapproximated by a side-plate behind the element mesh or grid 2 shown inFIG. 1, which connects the medial and lateral cortical bone. Thisside-plate has the material properties of cortical bone.

The design criterion for the prosthesis 1 in FIG. 1 for the applicationof the procedure of FIG. 2 is as follows:

Shear and normal stresses as low as possible.

A uniform distribution of the shear stresses.

A uniform distribution of the normal stresses.

No tensile stresses between cement and cortical bone.

Limited tensile stresses between cement and trabecular bone.

Low strain energy density in the cement near the interface.

The geometry (FIGS. 1 and 3) thus found hardly shows sensitivity for thenature of the external load (moment or force) on the head 25 of theprosthesis. However, the material of the prosthesis 1, 20 (stainlesssteel, CoCrMo, titanium) does significantly change the design. Thepresent examples are based on the selection of CoCrMo as prostheticmaterial.

The distinct features of the design shown in FIG. 3 can be described asfollows:

a metal stem 22 with a proximal taper;

the stem has a distal taper;

after insertion of the stem into the bone the stem tip must be locatedmedially of the intramedullary axis;

the cement mantle (FIG. 1) has at the medial side at the resection planea thickness of ±10 mm;

the cement mantle at the medial side, from 11/2 cm beneath the resectionplane towards distally, displays a slowly increasing thickness from 2 to41/2 mm; from halfway down unto the stemtip the cement mantle on themedial side remains 41/2 mm thick;

the thickness of the cementmantle at the lateral distal side increasesdownwards unto the stem tip.

The prostheses 20 (FIG. 3) consists of a stem 22, a neck 23, and a cone24, on which a femoral head 25 can be positioned. This modular systemgives the possibility of choosing a variable neck length and a variablematerial for the femoral head (possibly including ceramic or otherartificial material). From FIG. 3 it can be seen further that the distallateral side of the prosthesis 21 is plane, while ending upwards in awidened and rounded back 19, which contains flanges or ribs on theanterior and prosterior sides. These flanges or ribs produce astiffening of the proximal stem and cause a favourable stressdistribution in the cement mantle. A characteristic length between thedistal stem tip and the intersection of the neck axis and theintermedullary axis amounts to 125 mm. As the drawings are made toscale, other characteristic dimensions can be read from FIG. 3.

With a total of 5 metal prostheses 34-38 (FIG. 4) a patient populationcan be provided with a well fitting prosthesis in almost all cases; theprostheses 34-38 differ in their dimensions at the medial proximal sideonly. With one of the prosthesis 34-38 a series of bones can be served,in order to approximately create the stress patterns afore mentioned.

An acetabular prosthesis 26 (FIG. 3) designed in accordance with thepresent invention consists of an insert 29 out of UHMWPE (Ultra HighMolecular Weight Polyethylene) and a metal backing 28. The geometrywhich is shaped in accordance with the same criteria as described forthe femoral head prosthesis can be seen in FIGS. 3 and 5 and can bedescribed as follows:

a rim 30 to ensure a constant cement thickness of 3 mm;

a metal backing of nonuniform thickness;

a metal backing which continues at the lateral superior side unto theacetabular outer plane;

a metal backing which continues at the medial inferior side unto theacetabular plane as far as possible;

a metal backing which is nowhere thicker than 1 mm at the periphery;

a metal backing which has its maximal thickness of for example 5 mm atits center or in its near surrounding.

The outside diameter of the metal backing (FIG. 5A-5G) is in between 46and 58 mm including the cement mantle. A femoral head diameter of 28 mmand a minimal polyethylene thickness of 5 mm has been chosen. All otherdimensions are listed in table 1.

                  TABLE 1                                                         ______________________________________                                        Acetabular cup dimensions in mm for 7 sizes.                                                                         position                                                               central                                                                              headcenter                             outside                                                                             cement-  periferal                                                                              central poly-  outside ace-                           dia-  mantle   backing  backing ethylene                                                                             tabular                                meter thickness                                                                              thickness                                                                              thickness                                                                             thickness                                                                            plane                                  ______________________________________                                        46    3        1         11/2    51/4  3/4                                    48    3        1        2        51/2  1/2                                    50    3        1         21/2    53/4  1/4                                    52    3        1        3       6      0                                      54    3        1        4       6      0                                      56    3        1        5       6      0                                      58    3        1        5       7      0                                      ______________________________________                                    

The cup is also provided with spacers 64, which are mounted in holes 27in the metal backing 28, to ensure uniform thickness of the cementmantle at insertion. The polyethylene insert 29 is mounted into themetal backing 28 by means of recessions and studs 32. The insert isprovided with a rim 30 which is provided with a groove 90 to contain arim 31 of the metal backing 28. The rim 30 serves to position of theacetabular prosthesis in the reamed pelvis and to enable pressurizationof cement during the operation, as will be described hereafter. Themound of the polyethylene insert is positioned at an angle of 10 degreesto the acetabular plane, to prevent luxation of the femoral head 25.

When the geometry of a femoral head prosthesis has been shapedaccurately as described above, also considering the shape of the cementmantle, such a prosthesis 38 (FIG. 6) has to be positioned accurately.However, also a prosthesis not designed according the method mentionedabove can be positioned with the method described hereafter. Afterresection of the femur at plane R (FIG. 4) the intramedullary canal ofthe femur is reamed cylindrically. By means of a rasp with a smoothdistal stem serving as a guide in the drill hole, the proximal femur israsped to size accurately. With the rasp itself in situ as a mould, anaccurate recession is made to contain a proximal spacer 51.

The prosthesis 38, provided with two spacers 51 and 48 (FIGS. 3,6) ispositioned into the bone 47. The first spacer 51 is positioned at thelevel of the resection plane on the proximal stem. Apart from an opening91 for the prosthesis, the proximal spacer 51 contains a hole 55 for theairvent tube 53, and a hole 54 for the injection of bone cement. Theproximal spacer 51 in this way positions the proximal stem and blocksthe femoral canal 92, which must be cemented.

The second spacer 48 fits in the cylinderically drilled diaphysis 34with minimal play, ensures an accurate positioning of the stem tip withrespect to the bone 47, and also seals the lower side of the femoralcanal 37. The distal spacer 48 is provided with a number of canals 52over its length, which are interconnected by a circular groove 49.

During cementing, an airvent tube 53 is installed with one end in one ofthe canals 52. After bushing and rinsing of the intramedullary space,the assembly of the prosthesis 38, spacers 58 and 51, and airvent tube53 is inserted. The remaining intramedullary space is thus cementedsecondarily. Through the filler opening 54, cement is injected in thefemoral canal 37; for example by means of a cement gun (not shown here)with small nozzle diameter. After filling of the canal the cement can bepressurized in order to penetrate into the canals 52 of the distalspacer and into the bone 47. Cement injection can also take placethrough a hole to be drilled through a part of the bone structure.

The spacers 48 and 51 are preferably made of PMMA(polymethylmethacrylate) in order to form a firm bond with the curedcement. After pressurizing the bone cement for a short period of time,the airvent tube 43 can be retrieved slowly, while the cement mass iskept pressurized. The space remaining after removal of the airvent tube53 will be filled with cement.

The airvent tube 53 serves for the drainage of air, fluids and bonemarrow which is pushed down the femoral canal 37 by the cement duringcementing. In order to achieve a firm interlock between the cement andthe bone, the cement can also be kept pressurized after the removal ofthe airvent tube 53.

Although the method described here refers to a femoral head prosthesis,it can also be applied for the positioning and fixation of all otherprostheses, provided with a stem to be positioned in a long bone. Theopening 50 in the distal spacer 51 is not circular and is locatedexcentrically; when applied to another prosthesis this opening 50 canfor example be circular or be located centrally.

The second embodiment of a femoral head prosthesis according to thepresent invention (FIGS. 7, 8 and 9A-9F) can by means of the bent shapedetermined by the dimension K relative to L and W, more easily beremoved from a bone, which is important for revision operations.

Further the shown shape lacks the flenches from the first embodiment,providing more space for the applying of a cement mantle.

Preferably K has a value of 11.25 mm relative to a dimension L of 135 mmand a dimension W of 41 mm, relating to a mid-size prosthesis.

A second embodiment of the acetabular prosthesis according to thepresent invention (FIG. 10) shows spacers 71, preferably ofpolyethylene, to be inserted from the outside into backing 72. Insert 73is provided with a fixed central element 74 to be inserted in centralhole 76. The insert 73 is provided with a groove 77 disposed in a rim 78provided with three notches 79 equally spaced around the periphery ofinsert 73.

We claim:
 1. A method for determining the shape of a prothesis to befixed in a bone, comprising the steps of:selecting a model for theprothesis and the bone; choosing an initial shape of the prosthesis forthe model; presetting a desired pattern of stress between the bone andthe prosthesis; setting one or more external load conditions in themodel; calculating from the model a stress pattern between the bone andthe prosthesis for the selected initial shape of the prosthesis;selecting a design criterium for the difference between the calculatedstress and desired stress pattern for the model; and iterativelychanging the shape of the prosthesis for the model, such thatdifferences between the calculated stresses of successive shapes of theprosthesis and the desired stress pattern are reduced to the designcriterium.
 2. A method according to claim 1, wherein selecting the modelfurther comprises incorporating a cement mantle and its materialproperties into the model and in which the strain energy density actinginside the cement mantle is changed until its value reaches the designcriterium.
 3. A method according to claim 1, in which selecting thedesign criterium includes the step of minimizing normal and shearstresses acting between the prosthesis and the bone.
 4. A methodaccording to claim 1, in which the iteratively derived shape of theprosthesis includes nodal points for establishing dimensional contouringof the prosthesis shape thereat and the method further includesinterpolating between nodal points for obtaining a smooth contour forthe prosthesis.
 5. A method for making a prosthesis to be inserted intothe canal of a resected long bone comprising the steps of:selecting aninitial shape for the prosthesis; establishing predetermined allowablestress pattern values between the prosthesis and the canal of the longbone; providing a model of the initial shape of the prosthesis asinserted into the canal of the long bone; determining stress patternsbetween the initial shape of the prosthesis and the long bone inaccordance with the model; iteratively modifying the initial shape ofthe said prosthesis until the stress patterns between the modifiedprosthesis and the canal of the long bone are reduced to said allowablepredetermined values.
 6. The method as defined in claim 5, whichcomprises making a set of shaped prostheses to fit a group of differentusers.
 7. A method of shaping a prosthesis to be fixed in a bone foroptimizing the stresses generated at the prosthesis-bone interfacecomprising the following steps:selecting an initial shape of theprosthesis; presetting a desired pattern of stresses along the nodes ofa grid, said nodes defining points along the prosthesis-bone interface;setting at least one external load condition; calculating the stressesat said nodes for said initial shape of said prosthesis; calculating thedifferences between said calculated stresses and said desired pattern ofstresses; iteratively changing the shape of said prosthesis such thatthe differences between the calculated stresses of successive shapes ofsaid prosthesis and said desired pattern of stresses are reduced to apredetermined value.